The present invention relates generally to liquid crystal displays and more particularly to an improved liquid crystal display exhibiting enhanced optical viewing performance.
Liquid crystal displays, which were first introduced in the 1970's and have since been used in watches, clocks, pocket calculators, portable personal computers, navigational instruments and the like, are electronically switched displays that make use of changes in the reflective properties of liquid crystals present in an electric field. A typical liquid crystal display comprises a liquid crystal display panel, said panel comprising a thin film of liquid crystals sandwiched between a pair of transparent electrodes. The ruggedized liquid crystal display panel is typically positioned between a pair of glass plates, the glass plates being sealed together around their respective edges. The glass plates may be assembled with spacers between them to maintain a constant separation distance. Two crossed axis polarizers are adhered to the respective inside surfaces of the glass plates, one polarizer also being adhered to the front of the liquid crystal display panel and the other polarizer also being adhered to the rear of the liquid crystal display panel. When a voltage is applied selectively across the electrodes, the liquid crystal molecules between them are rearranged or switched in polarization so that light is either reflected or absorbed in the region bounded by the electrodes to form characters or graphics. Many liquid crystal displays include a backlight for night or dark viewing, and many liquid crystal displays include a layer of indium tin oxide or another suitable material positioned between the front polarizer and the front glass plate for use as an electromagnetic interference shield. Some liquid crystal displays intended for use in cold environments also include a layer of indium tin oxide or another suitable material positioned between the rear polarizer and the rear glass plate and electrically connected to a power source for use as a heating element.
Some of the advantages of liquid crystal displays, as compared to other digital displays, are their comparatively low voltage and power requirements. Unfortunately, however, one of the principal disadvantages of liquid crystal displays is that, due to their low contrast and a "washed-out" effect caused by back-reflected ambient light seen by a viewer, they cannot easily be read under high ambient light conditions, such as when placed in direct sunlight.
One approach which has been taken to improve the viewing performance of liquid crystal displays, particularly when viewed under high ambient light conditions, has been to apply an anti-reflection coating to the front of the display. Although such an approach has provided some improvement to the viewing characteristics of liquid crystal displays by reducing the amount of back-reflected light seen by the viewer, further improvement is greatly needed.
A few patents in the field of liquid crystal display technology are described below.
In U.S. Pat. No. 5,570,214, inventors Abileah et al., which issued Oct. 29, 1996, and which is incorporated herein by reference, there is disclosed a liquid crystal display comprising a plurality of pixels, each of the pixels being comprised of a pair of driving electrodes and a twisted nematic liquid crystal material located therebetween. The liquid crystal material is of a thickness "d" and has an anistrophy .DELTA.N such that the product of d.multidot..DELTA.N is about 400-550 nm, and the liquid crystal material is capable of twisting at least one normally incident visible wavelength of light passing therethrough in an amount of about 80.degree.-100.degree.. The device also comprises a rear, light-entrance polarizer having a transmission axis oriented in a first direction, a front, light-exit polarizer having a transmission axis oriented in a second direction with respect to the first direction thereby to define a normally white display, a rear retardation film disposed between the rear polarizer and the twisted nematic liquid crystal material, and a front retardation film disposed between the front polarizer and the liquid crystal material. The transmission axes of the retardation films are so arranged each with respect to the others so as to achieve a white light contrast ratio of at least about 10:1 over a horizontal angular span of at least about 100.degree. and over a vertical angular span of greater than about 55.degree..
In U.S. Pat. No. 5,594,568, inventors Abileah et al., which issued Jan. 14, 1997, and which is incorporated herein by reference, there is disclosed a normally white liquid crystal display comprising a rear, light-entrance polarizer having a transmission axis oriented in a first direction; a front, light-exit polarizer having a transmission axis oriented in a second direction with respect to the first direction so as to define a normally white display; a twisted nematic liquid crystal layer; and first and second positively birefringent uniaxial retardation films both disposed between the twisted nematic liquid crystal layer and one of the rear, light-entrance polarizer and the front, light-exit polarizer. The liquid crystal layer in the off state twists at least one normally incident visible wavelength of light less than about 110.degree. as it passes therethrough, and the first and second uniaxial retardation films have retardation values of from about 80-200 nm. The transmission axes of the polarizers and optical axes of the retardation films are so arranged each with respect to the others so as to achieve high contrast ratios over a large range of predetermined viewing angles.
In U.S. Pat. No. 5,706,068, inventors Abileah et al., which issued Jan. 6, 1998, and which is incorporated herein by reference, there is disclosed a liquid crystal display comprising a plurality of pixels, each of the pixels being comprised of a pair of driving electrodes and a twisted nematic liquid crystal material located therebetween. The liquid crystal material is of a thickness "d" and has an anistrophy .DELTA.N such that the product of d.multidot..DELTA.N is about 400-550 nm, and the liquid crystal material is capable of twisting at least one normally incident visible wavelength of light passing therethrough in an amount of about 800.degree.-100.degree.. The device also comprises a rear, light-entrance polarizer having a transmission axis oriented in a first direction, a front, light-exit polarizer having a transmission axis oriented in a second direction with respect to the first direction thereby to define a normally white display, a rear retardation film disposed between the rear polarizer and the twisted nematic liquid crystal material, and a front retardation film disposed between the front polarizer and the liquid crystal material. The transmission axes of the retardation films are so arranged each with respect to the others so as to achieve a white light contrast ratio of at least about 10:1 over a horizontal angular span of at least about 100.degree. and over a vertical angular span of greater than about 55.degree..
In U.S. Pat. No. 5,739,881, inventors Xu et al., which issued Apr. 14, 1998, and which is incorporated herein by reference, there is disclosed a normally white twisted nematic liquid crystal display comprising a liquid crystal layer for twisting at least one normally incident visible wavelength of light from about 80.degree. to 100.degree. as it passes therethrough when the liquid crystal layer is in substantially the off-state thereby defining a twisted nematic display. The display also comprises a pair of negative retarders sandwiching the liquid crystal layer therebetween and a pair of positive retarders sandwiching both the liquid crystal layer and the negative retarders therebetween, wherein the retardation value of each of the negative retarders is from about -60 to -200 nm and the retardation value of each of the positive retarders is from about 80 to 200 nm.